The cerebellum is essential for the control of movement and plays an important role in cognitive processes. Within the cerebellum, estrogens have long been demonstrated to affect synaptic circuitry and function. Yet to date, the mechanisms by which estradiol can alter cerebellar output have remained a mystery. Using state-of-the-art optical imaging techniques, we have found estrogen receptor signaling to be essential for the maintenance of the parallel fiber synapses connecting cerebellar granule cells to Purkinje neurons. Further, we have found that estrogen receptor regulation of the parallel fiber synapse to be dependent on locally synthesized estradiol. These effects are observed in both male and female mice. Since parallel fibers dominate the cytoarchitecture of the cerebellar cortex, and are central elements in modulating cerebellar function, we believe these data are the beginnings to our understanding of a new fundamental mechanism of estrogen action. Using the combined expertise and experiences of two complementary investigators at the University of Minnesota, our goal is to characterize this phenomenon. Specific Aim 1 will determine whether electrical activity within the parallel fiber-Purkinje cell network drives local neuroestrogen production. Specific Aim 2 will identify the estrogen receptor(s) responsible for maintaining the efficacy of synaptic neurotransmission between the cerebellar granule neurons and Purkinje cells. This Aim will also determine whether the estrogen receptors are located pre- or postsynaptically. Specific Aim 3 will evaluate the functional impact of diminishing estrogen signaling on cerebellar-mediated locomotor behavior. By characterizing the critical regulatory mechanisms for estrogen receptor support of cerebellar glutamatergic neurotransmission, we will have uncovered a heretofore unknown aspect of nervous system function. In addition, these results will have important implications in the use of estrogen receptor modulators to benefit health and its impact in the treatment of disease.